Paper

Combining first-principles calculations, a pressure-dependent two-orbital model, self-consistent FLEX calculations, and the linearized Eliashberg equation, this study reveals the microscopic mechanism of pressure-enhanced and robust superconductivity in infinite-layer La₀.₈Sr₀.₂NiO₂. The results show that increasing pressure enlarges the kinetic energy scale, reduces the effective correlation strength U/t, enhances interlayer hybridization, and transfers holes from the La/Sr charge reservoir to the correlated Ni region; at low pressure, the enlarged kinetic energy scale and the approach to optimal intermediate coupling promote pairing, whereas at high pressure, pressure-induced self-doping drives the system into the overdoped regime and suppresses superconductivity, thereby forming a broad superconducting dome. Although compression significantly three-dimensionalizes the Fermi surface, the spin susceptibility relevant to pairing depends weakly on q_z and still peaks mainly near (π,π), so the Ni d_{x²−y²}-dominated d-wave pairing state remains stable over the calculated pressure range. This constrained low-energy pairing framework explains the unusual robustness of superconductivity in this material under megabar-level compression.

Addressing the contradiction in the bilayer nickelate La3Ni2O7 between a bulk nodeless energy gap and d-wave-like features in point-contact transport, the paper theoretically assumes that interlayer pairing driven by strong interlayer antiferromagnetic coupling is the dominant pairing channel and introduces interlayer and intralayer pairing amplitudes in a two-orbital tight-binding model to reproduce the multi-gap structure seen by ARPES/STM. It is pointed out that when point-contact coupling to the two layers is inhomogeneous, a contact potential that is odd under layer-exchange mirror symmetry can act as an effective Zeeman field on the layer pseudospin, thereby binding in-gap Andreev states in the barrier region that shift with barrier width and strength; in the mirror eigenbasis, this barrier mixes mirror sectors with opposite pairing sign and opens a sign-reversal Andreev channel. Simulations for a top-layer barrier on La3Ni2O7 show that its subgap conductance can reproduce the orientation-dependent zero-bias conductance peaks and side peaks usually taken as a d-wave fingerprint, indicating that a nodeless interlayer pairing state can mimic d-wave Andreev signals and providing a natural picture for reconciling conflicting experimental evidence.

This study employs ab initio and functional renormalization group calculations to investigate the effects of electron-phonon coupling on magnetism and superconductivity in trilayer nickelates. The authors identify a significant electron-phonon coupling in the out-of-plane oxygen breathing mode and construct a symmetry-resolved, layer-dependent coupling. The relative phase among the three layers generates different retarded electron interactions, thereby mode-selectively enhancing or suppressing spin density wave order and s±-wave superconductivity. The mechanism involves a screening-mediated spin-phonon interaction spanning multiple energy scales: electron-phonon coupling first alters high-energy charge screening, then reshapes low-energy magnetic fluctuations, and ultimately affects pairing. The selected mode yields a negative isotope coefficient for the magnetic state, qualitatively consistent with experiments. These results reveal how interlayer phonons regulate electronically driven ordered states and provide a microscopic framework for understanding isotope effects in trilayer nickelates.

This study addresses bulk La6Ni5O12 by employing a five-layer Hubbard model and, within the random phase approximation (RPA) framework, examines the influence of electron-phonon coupling on spin-fluctuation-induced d_{x^2-y^2}-wave pairing; the dominant pairing state given by this framework is d_{x^2-y^2}-wave. Two phenomenological couplings inspired by oxygen breathing modes, when present alone, favor s-wave pairing, but their attractive scattering between regions where the spin-fluctuation-induced gap has opposite signs suppresses d-wave pairing. Upon inclusion of electronic charge screening, this suppression is significantly weakened; comparison of the s-wave and d-wave pairing strengths with and without screening shows that screening shifts their boundary toward the side favorable to d-wave pairing, and momentum-resolved projections further identify where the unfavorable scattering processes are weakened. Under representative electron interactions, d-wave pairing remains dominant; these results emphasize the competing roles of the momentum structure of electron-phonon coupling and charge screening in spin-fluctuation d-wave pairing.

We report a 139La-NMR study of polycrystalline samples of multi(n)-layered nickelates, La3Ni2O7−δ (n = 2) and La4Ni3O10−δ (n = 3), at ambient pressure. Measurements of the nuclear magnetic resonance (NMR) spectra and nuclear spin relaxation rate (1/T1) indicate the emergence of a density wave order with a gap below T* ∼ 150 K for La3Ni2O7−δ and ∼130 K for La4Ni3O10−δ. The finite value of 1/T1 below T* indicates metallic ground states with the remaining density of states at the Fermi level (EF) under the density wave order. These features are attributed to multiple d electron bands with different characteristics. Above T*, the gradual decrease in 1/T1T upon cooling implies the presence of a band with flat dispersion near EF. From our microscopic probes, we point out that these nickelates (n = 2 and 3) possess similar electronic states despite the difference in the formal valence of the Ni d electron states, which provides a basis for understanding the novel high-Tc superconductivity under high pressures.

By combining linear terahertz time-domain spectroscopy with third-harmonic generation, this study systematically probes the superconducting pairing symmetry and normal-state pseudogap in compressively strained (La,Pr)₃Ni₂O₇ thin films. Linear terahertz spectroscopy reveals a significant suppression of low-frequency spectral weight below the superconducting transition temperature, accompanied by a weak coherence peak and a large residual conductivity persisting down to near-zero temperature, consistent with a disordered s±-wave pairing scenario. The nonlinear third-harmonic signal sharply enhances upon entering the superconducting state, but its response persists above the superconducting transition temperature, exhibiting a kink at approximately 100 K, which is attributed to the normal-state pseudogap based on similar temperature scales observed in angle-resolved photoemission spectroscopy on analogous films. This study establishes (La,Pr)₃Ni₂O₇ as a bulk superconductor with s±-wave-like pairing, where superconductivity coexists and likely competes with another ordered state, providing a new platform for exploring unconventional superconducting mechanisms beyond cuprates and iron-based superconductors.

Here, combining X-ray absorption spectroscopy and resonant inelastic X-ray scattering, we investigate the multi-orbital electronic structure and magnetic excitations of the trilayer nickelate Pr4Ni3O10. Because the Pr resonances do not cover the Ni L3 edge, the Ni L3 edge can be probed directly; polarization-dependent measurements at the Ni L edge and O K edge resolve in-plane and out-of-plane Ni-O hybridization channels. Momentum-resolved RIXS reveals dispersive magnetic excitations that soften at the incommensurate spin-density-wave wavevector. Effective spin-wave analysis yields substantial intra-trilayer exchange coupling, comparable in magnitude to the dominant in-plane interaction and insensitive to rare-earth substitution. These results indicate that the multilayer structure, rather than rare-earth substitution alone, is the key factor tuning the magnetic interactions in this class of materials.

Using polarization-resolved Raman scattering combined with a two-orbital model calculation, this study reveals the multi-orbital character of the density wave in the trilayer nickelate La4Ni3O10. In the phonon sector, the oxygen bond-stretching mode exhibits a pronounced intensity anomaly near the density-wave transition temperature T_DW, while the phonon line shape remains essentially symmetric with negligible changes in frequency shift and linewidth, indicating a moderate electron-phonon coupling and suggesting that the density wave is primarily driven by electronic instability rather than by the lattice. In the electronic Raman scattering, as temperature decreases, the spectral weight of the continuum is significantly depleted below ∼114 meV (∼910 cm⁻¹), and a polarization-dependent sharp peak emerges at this energy; this peak is strongest in the B₂g channel and nearly absent in the B₁g channel. By constructing a two-orbital density-wave state model that simultaneously includes Ni-3d_{x²−y²} and Ni-3d_{z²} orbitals and retains the Raman vertices in the orbital space, calculations confirm that this peak corresponds to the density-wave energy gap 2Δ_DW, whose opening displays incoherent and non-mean-field behavior, and that the spectral response necessarily relies on the mixed contributions of both orbitals and cannot be reduced to any single-orbital projection. These results demonstrate that the density-wave instability in La4Ni3O10 has an intrinsic multi-orbital origin, providing crucial experimental and theoretical insights for understanding its competition or coexistence with superconductivity.

Nature of charge density waves and metal-insulator transition in pressurized La₃Ni₂O₇

By simultaneously measuring the resistance and diamagnetism of (La,Pr)3Ni2O7 thin films and mapping the vortex-melting phase diagram under in-plane and out-of-plane magnetic fields, we investigated the anisotropy of superconductivity in bilayer nickelates. For H ∥ c, geometric confinement in ultrathin films leads to pancake vortices; the vortex-melting-field anisotropy parameter γ_Hm = H_m^ab/H_m^c decreases monotonically with decreasing temperature and approaches 1 at low temperatures, indicating the formation of a nearly isotropic vortex solid. Within anisotropic Ginzburg-Landau scaling, this ratio reflects the anisotropy of the superfluid density, and the results therefore point to a nearly isotropic superfluid response; this is incompatible with a contribution solely from the strictly two-dimensional Ni 3d_{x^2-y^2} band, but is naturally explained by a significant interlayer superfluid weight provided by the Ni 3d_{z^2}-derived γ band. This work provides thermodynamic evidence that superconductivity in bilayer nickelate thin films includes a substantial contribution from the γ band.